Debra goes with Dexter to a jewelry store, to help him buy a wedding ring for Rita.

When she tells him that it should match the engagement ring, he tells her that Rita told him she didn't want an engagement ring.

This shocks Debra, who berates Dex for being an idiot (for having believed Rita), and tells him to buy both rings.

She
also asks him for advice on her own love life (with Anton), pretending
that it's just a hypothetical question. But when Dexter tells her
the obvious, it isn't what she wants to hear, so she gets angry and
exits.

Since it was strictly an interior shot, with no view of
the store's exterior, I had no way of knowing where it was.

My
hunch
was that it might be on Second Street, the main shopping boulevard of
Belmont Shore (in Long Beach), since the script mentions the store
being on Second Street, and they film often in Long Beach. But
that turned out to be a false lead. As usual, they misled us
about the actual address.

It
wasn't until six years later, in late 2014, that I was contacted by J
Ryan, a new Dexter fan, who recognized the store interior in the scene as his and my
wife's jeweler, Universal Jewelers.

I was able to easily confirm it by looking at photos on the store's website. ( Thanks, J Ryan! )